1. Field of the Invention
The present invention relates to an optical scanning apparatus which may be particularly employed by an image forming apparatus, such as a laser beam printer (LBP), a digital copier, or a multi-function printer, having an electrophotographic process. The present invention also relates to an image forming apparatus that uses the optical scanning apparatus.
2. Description of the Related Art
Hitherto, an optical scanning apparatus has been used for a LBP, a digital copier, a multi-function printer, or the like.
In the optical scanning apparatus, a light source modulates and emits a light beam in accordance with an image signal, and a light deflector, which is, for example, a rotatable polygonal mirror (polygonal mirror), periodically deflects the light beam.
Then, an imaging optical system (scanning optical system) having an fθ characteristic condenses the deflected light beam onto a surface of a photosensitive recording medium (photosensitive drum) in a spot-like form, so that the light beam scans the surface for image recording.
FIG. 21 is a schematic illustration showing a primary portion of an optical scanning apparatus according to a related art example.
Referring to FIG. 21, a light source 1 emits a single or plurality of divergent light beams. A collimator lens 2 converts the light beams into a single or plurality of parallel light beams. An aperture stop 3 limits the light beams. The light beams are incident on a cylindrical lens 4 having a specific refractive power only in a sub-scanning direction.
From among the parallel light beams incident on the cylindrical lens 4, light beams within a main-scanning cross section are directly exited from the cylindrical lens 4 without any change.
Light beams within a sub-scanning cross section are condensed and form linear images onto a deflecting surface (reflection surface) 5a of a light deflector 5 which is a polygonal mirror.
The light beams deflected by the deflecting surface 5a of the light deflector 5 are guided onto a photosensitive drum surface 8 serving as a surface to be scanned, through an imaging lens 6 having an fθ characteristic.
The light deflector 5 is rotated in a direction indicated by arrow A, so that the single or plurality of light beams scan the photosensitive drum surface 8 in a direction indicated by arrow B (main-scanning direction) to record image information.
Referring to FIG. 21, reference numeral 18 denotes a mirror for synchronism detection, and 19 denotes a sensor for synchronism detection.
As optical scanning apparatuses for color image forming apparatuses that form color images, various optical scanning apparatuses have been suggested in which a light deflector serving as a deflecting unit is shared by a plurality of light beams for reduction in size of the entire apparatus (for example, see Japanese Patent Laid-Open No. 2002-055293).
FIG. 22 is a schematic illustration showing a primary portion of an optical scanning apparatus for a color image forming apparatus according to a related art example in which a light deflector is shared by a plurality of light beams.
The optical scanning apparatus in FIG. 22 includes scanning units SR and SL disposed on both sides with a light deflector 5 interposed therebetween. Vertically arranged two light beams are obliquely incident on a single deflecting surface within a sub-scanning cross section.
In the scanning unit SR on the one side of the light deflector 5, a plurality of light beams deflected by the light deflector 5 scan two photosensitive drum surfaces 8A and 8B (surfaces to be scanned) in a uniform direction.
In the scanning unit SL on the other side, a plurality of light beams deflected by the light deflector 5 scan two photosensitive drum surfaces 8C and 8D in a uniform direction.
The scanning units SR and SL in FIG. 22 each use a common imaging optical system (scanning optical system) for vertically arranged light beams Ra and Rb.
In the imaging optical system, an optical path of the light beam Rb that forms an image onto each of the photosensitive drum surfaces 8B and 8C located close to the light deflector 5 is reflected with the use of three reflection mirrors, so as to prevent the light beam Rb from interfering with optical components such as a lens and a mirror.
When the number of reflection mirrors is increased, stripes may likely appear in an image due to dusts adhering to the mirrors and scars of the mirrors.
In addition, banding of scanning lines due to vibration of the mirrors may become noticeable. Further, as the number of mirrors is increased, the entire apparatus may be complicated.
Thus, it is desirable to form the optical scanning apparatus with a minimum number of reflection mirrors.
FIG. 23 is a sub-scanning cross section of the optical scanning apparatus in FIG. 22 when the number of reflection mirrors is two in the imaging optical system SB that forms images onto the photosensitive drum surfaces 8B and 8C located close to the light deflector 5 (i.e., that scans the photosensitive drum surfaces).
It is found that the light beams Ra that form images onto the photosensitive drum surfaces 8A and 8D located far from the light deflector 5 interfere with imaging lenses for forming images onto the photosensitive drum surfaces 8B and 8C located close to the light deflector 5.
To avoid this, the positions of the imaging lenses and the reflection mirrors may be changed. However, it is difficult to attain this within a predetermined limited space of a main body of the color image forming apparatus.
Thus, as shown in FIG. 23, the number of reflection mirrors is increased, and the optical path is properly reflected in the given space in the known configuration, although problems such as stripes in an image and banding remain.
Japanese Patent Laid-Open No. 2002-055293 discloses an optical scanning apparatus capable of saving the space by using different imaging optical systems for light beams that form images onto different surfaces to be scanned.
FIG. 24 is a sub-scanning cross section disclosed in Japanese Patent Laid-Open No. 2002-055293.
In the drawing, an optical scanning apparatus is illustrated, in which a single reflection mirror is used in each of the imaging optical system SB that forms images onto the photosensitive drum surfaces 8B and 8C located close to the light deflector 5 and the imaging optical system SA that forms images onto the photosensitive drum surfaces 8A and 8D located far from the light deflector 5.
In the known optical scanning apparatus for the color image forming apparatus, the following problems are present.
Referring to FIGS. 22 and 23, in the known configuration using the common imaging optical system for the vertically arranged light beams Ra and Rb, the freedom of arrangement of optical components is restricted. The number of mirrors is increased as shown in FIG. 22, and interference of light beams with optical components occurs as shown in FIG. 23.
In particular, the above-mentioned problems may occur when an imaging optical system has a configuration in which the imaging lens located closest to the photosensitive drum surfaces 8A and 8D in the imaging optical system SA that forms images onto the photosensitive drum surfaces 8A and 8D located farthest from the light deflector 5 is disposed closer to the light deflector 5 than the reflection mirror located closest to the photosensitive drum surfaces 8A and 8D is.
In contrast, the optical scanning apparatus disclosed in Japanese Patent Laid-Open No. 2002-055293 having the different imaging optical systems SA and SB for the plurality of light beams has the configuration in which the optical path is reflected by the single reflection mirror in each of the imaging optical systems SA and SB.
Accordingly, a large difference, which is as large as a distance between the surfaces to be scanned, is generated between an optical path length of the imaging optical system SB that forms images onto the photosensitive drum surfaces 8B and 8C located close to the light deflector 5 and an optical path length of the imaging optical system SA that forms images onto the photosensitive drum surfaces 8A and 8D located far from the light deflector 5.
Herein, an optical path length is an optical distance from a deflection point of a light deflector to a surface to be scanned. Also, in the specification, the term “optically” represents “in a condition where an optical path is developed”.
Since the light beam is incident on a plane perpendicular to the deflecting surface of the light deflector 5 within the sub-scanning cross section, the optical system has to sufficiently correct deformation of a spot as a result of torsion of wavefront aberration.
However, in the two imaging optical systems SA and SB, in which the imaging lens located close to the light deflector 5 is shared, and the large difference as the distance between the surfaces to be scanned is present, it is difficult to correct the torsion of wavefront aberration and to satisfy other paraxial performances.